Through mechanistic studies of diverse reactions, it is proposed to explore areas in which the conventional framework for understanding organic reactions is not adequate. A critical mass of work in the literature, both from our group and others, is supporting a greatly expanded importance for dynamic effects in the chemistry of ordinary organic reactions in solution. When a reaction involves a potential energy surface having two adjacent saddle points, the product of the reaction is decided by dynamic effects that are unrelated to any barrier for formation of the possible products. It is proposed to examine a variety of important reactions that are likely to involve dynamic effects in order to understand and ultimately control their selectivity. It is also proposed to use mechanistic studies to bring the understanding of reaction selectivity, particularly entropic effects on selectivity, to the level where highly accurate predictions of product ratios are possible. Finally, mechanistic studies are proposed with the aim of understanding and controlling selectivity in some important new reactions. The health-relatedness of this work derives from its impact on the understanding of reactions important in the synthesis of medicinally important substances and reactions important in biosynthetic pathways.